In many biomedical applications such as cell encapsulation and implantations (R. Langer, Adv. Mater. 2009, 21, 3235,) the biological response of the body to the implanted medical device and biomaterial play a critical role in determining the long term success of the implant (J. M. Anderson, Annu. Rev. Mater. Res. 2001, 31, 81; M. T. Novak, J. D. Bryers, W. M. Reichert, Biomaterials 2009, 30, 1989; D. F. Williams, Biomaterials 2008, 29, 2941.) Often, the acute and chronic inflammatory response caused by foreign material and the formation of fibrotic tissue around the device lead to compromised function, device failure, or medical complications. The so-called foreign body response consists of a series of complex reactions involving various types of cells, chemokines, and cytokines. In the simplest view, the recruitment of inflammatory cells such as neutrophils and macrophages to the implantation site is characteristic of the early response, i.e., acute inflammation, while fibrosis is typically considered the end result of chronic inflammation. Both the physical and chemical properties of the biomaterial influence the intensity and/or duration of the foreign body response. Significant research efforts have thus been devoted to controlling the foreign body response by tuning material properties and surface chemistries (B. D. Ratner, S. J. Bryant, Annu. Rev. Biomed. Eng. 2004, 6, 41; W. K. Ward, J. Diabetes Sci. Technol. 2008, 2, 768; Y. Onuki, U. Bhardwaj, F. Papadimitrakopoulos, D. J. Burgess, J. Diabetes Sci. Technol. 2008, 2, 1003.)
Cationic polymers are an important class of polymers that have been used extensively in biomedical applications. For example, in cell encapsulation applications, a polycation is often used as a component to form a semi-permeable polyelectrolyte complex capsule, which protects the encapsulated cells from the attack of host immune systems but allows the facile diffusion of nutrients (T. M. S. Chang, Nature Reviews Drug Discovery 2005, 4, 221.) Poly(L-lysine) (PLL) was the first polycation used in alginate-based islet encapsulation system. It has since been widely used, but recently it was shown to have biocompatibility issues (G. Orive et al., Nature Medicine 2003, 9, 104.) Discovery of alternative, more biocompatible polycations therefore represents a critical step towards the final clinical success for cell encapsulation. Another example where polycations are used is in layer-by-layer (LBL) coatings, which has become one of the most versatile and robust surface modification approaches for various biomedical applications (T. Boudou, T. Crouzier, K. Ren, G. Blin, C. Picart, Adv. Mater. 2010, 22, 441.) The LBL coating involves alternating adsorption of complementary materials from solution, for example, positively and negatively charged species or polymers with hydrogen bond donor and acceptor groups (G. Decher, Science 1997, 277, 1232.) The diversity of cationic polymers used in LBL coatings have been relatively limited, with poly(allylamine hydrochloride) (PAH) being commonly used. Thus, development of new polycations, in particular, those with improved immuno-compatibility, will have a tremendous impact in biomedical applications.